This application relates generally to rotary machines and more particularly, to systems and methods for sealing a rotary machine.
At least some rotary machines, such as steam turbine engines, have a defined fluid flow path extending therethrough. The flow path includes, in a serial-flow relationship, a fluid inlet, a turbine, and a fluid outlet. Some rotary machines use a plurality of sealing assemblies in the flow path to facilitate increasing the operating efficiency of the rotary machine. Generally, known seal assemblies are coupled between a stationary component and a rotary component to provide sealing between a high-pressure area and a low-pressure area. Several known sealing assemblies include labyrinth teeth, flexible members such as brush seals, and hydrodynamic face seals.
At least some brush seals include tightly-packed, generally cylindrical bristles that are positioned adjacent to the rotary component and that are arranged in a staggered arrangement to facilitate reducing leakage. Generally, known bristles have a low radial stiffness that enables them to move during a rotary component rotor excursion while maintaining a tight clearance during steady state operations. Some known brush seals, however, are generally effective only when a limited pressure differential exists across the seal. In addition, at least some sealing assemblies include a plurality of labyrinth teeth that extend outwardly towards the rotary component. During operation of some rotary machines, vibrations caused by rotation of the rotary component may cause the labyrinth teeth to contact the rotary component. Over time, the labyrinth teeth may become worn and become less effective, which may cause performance deterioration of the rotary machines.
In some known rotary machines, hydrodynamic face seals are used to facilitate reducing leakage of a pressurized fluid through a gap between two components. Hydrodynamic face seals generally include a mating (rotating) ring and a seal (stationary) ring. During operation, grooves in the rotating ring generate a hydrodynamic force that causes the stationary ring to lift or separate from the rotating ring such that a small gap is created between the two rings. A sealing gas flows via the gap between the rotating and stationary rings. However, because the gap formed between the two rings is small, at least some known hydrodynamic face seals are fabricated as single continuous rings, which limits the diameter of the seal rings, and as such, may limit their use. There are at least two primary reasons a single continuous ring is limited in size. One reason is that it is challenging to achieve the required sealing face flatness for rings over 12 inches in diameter. Another critical reason is that sealing face coning increases with the fourth power of diameter from a thermal gradient across the seal face. Face coning is one main failure mode for hydrodynamic face seals. In addition, in large steam turbine and gas turbine applications, there is an assembly problem with single continuous ring construction since such large turbines are assembled in upper and lower halves. Therefore, to use a face seal in large steam and gas turbines, one of the sealing rings must be segmented. Because the mating ring can be fabricated integrally with the turbine shaft, it is preferred to have the seal ring segmented. Some advantages of a segmented seal ring include that seal face flatness can be straightforwardly achieved, and face coning from a thermal gradient is largely eliminated because seal ring hoop-stress is cut off at the segment joints. However, if the stationary ring is segmented and coupled together, there may be a step over the segment joint that is larger than the film thickness since hydrodynamic face seals generally operate with a clearance of about 0.002 inches or less. Such limitations make at least some known hydrodynamic face seals unsuitable for use in rotary machines with large diameter rotating shafts.